If this is a standard video DVD, it will typically have AC3/Dolby Digital audio. Movie Edit Pro comes with a decoder for that, but you will have to activate it first. You can find the appropriate option under the "Help" menu of Movie Edit Pro => "Activate additional functions" => "Activate Dolby Digital Stereo decoder". Click on the online activation/ordering option and follow the on-screen instructions. (It's a free activation.) After the activation has been performed, you should be able to import DVD/VOB material with Dolby Digital sound.
This is basically how it works in the current version of Movie Edit Pro. It's a similar procedure in the previous version 12. If you're running a version older than that, there's no built-in Dolby decoder. You will have to download and install an external codec for Dolby decoding, e.g.: